OH CR4P!

Ammonium nitrate, a chemical compound with the formula NH₄NO₃, is a well-known (albeit controversial) fertilizer in the agricultural sector. It is also an oxidizing agent in explosives, and is the main ingredient of ANFO, a common industrial explosive. Unfortunately, the compound has a bad history; in addition to its association with terrorism, it has been the cause of many industrial disasters over the years.

(Granulated ammonium nitrate. Image Credit: Sri Amman Chemicals) -->

The first of these was at a BASF chemical plant in Oppau, a suburb of Ludwigshafen Germany. It was there that the unfortunate discovery of ammonium nitrate's explosion potential led to the deaths of over 500 people.

The Incident

It was September 21, 1921 at the BASF plant when a small dynamite charge set off a massive explosion in the storage of 4,500 tons of ammonium sulfate and ammonium nitrate fertilizer. The explosion was said to be equal to 1-2 kilotonnes of TNT, heard as a loud bang from Munich more than 300 km (186 miles) away. The blast created a 90 m by 125 m crater 19 m deep, and ripped through 80% of the homes in the town. Glass windows were destroyed by the pressure wave in a nearby town 30 km away. 561 workers and residents of the town were killed and about 2,000 were injured from the accident.

(Illustration of the ruins of Oppau after the explosion.)

The Cause

During the previous war (WWI), a shortage of sulfur necessary for ammonium sulfate fertilizer production resulted in the increased production of ammonium nitrate as a replacement. Concentrations of ammonium nitrate increased over time in the mixture of the two compounds. Consequently, the fertilizer mixture began to clog and compact under its own weight because of ammonium nitrate's hygroscopic nature (tendency to absorb and hold water). Workers resorted to small charges of dynamite to loosen the paste back into powder form, a tried and true method for non-explosive ammonium sulfate.

Analysts say that on the day of the explosion, poor or inconsistent mixture of the fertilizer had resulted in an abnormally large concentration of ammonium nitrate in sections of the stockpile.

Lessons Learned

The explosion at the Oppau plant is an example of the fallibility of inductive reasoning when health and safety are on the line. This thought process - that what worked in the past will work now and in the future - is not sufficient in potentially high-risk operations. When entering new territory (e.g. using new chemicals), it's never OK to assume things will still work the same without sufficient facts or tests to support these claims.

Though the workers at the site never got to learn for themselves, the Oppau disaster was a terrible revelation of the explosive power of ammonium nitrate. Unfortunately, many more ammonium nitrate explosions have followed in the years after Oppau. This includes the 1947 Texas City disaster, which might be the most infamous explosion involving ammonium nitrate; not to mention the deadliest industrial accident in U.S. history.

References

The 1921 Oppau Disaster and its Aftermath - H2G2

Oppau Explosion - Wikipedia